In the conveyance of gases from coal conversion processes, e.g. coke oven gas, through gas lines, deposits are formed which tend to fall out in places such as valves, constrictions or places where change of direction of flow occurs. Whenever sufficient buildup occurs, a valve cannot be opened or shut and so becomes useless. It must then be removed and cleaned. In many cases, the valve cannot be used for any control function whatever and cannot be shut off for any reason even an emergency.
Other facilities such as small metering devices, pipelines, etc. also get plugged or flow restricted because of these deposits.
While much of the discussion which follows talks about coke oven gases, it will be understood that the invention described herein applies equally well to other gases having similar characteristics, and particularly to gases derived from coal conversion processes.
Prior to the installation of certain gas cleaning and separation processes, the downstream gas lines were cleaned by mechanical means. A mechanical cleaning device, called a "pig", was sent through the downstream gas lines. Generally, the material which was pushed out was tarry in nature and no doubt acted as a lubricant for the mechanical "pig". With the introduction of the gas cleaning and separation processes, tar could no longer pass in the downstream gas lines as all of the intervening steps resulted in a tar free gas. Concurrently, the gas became drier and cleaner because of the process. However, certain reactions continued to occur in the gas mains and in the underfiring system resulting in deposits being formed in various sections of the pipeline.
As time progressed, significant buildup of deposits in the down-river gas mains were found. The deposits caused flow restriction, were corrosive and resulted in additional weight on support structures. On several occasions it was necessary to cut the lines open, remove the deposits by drag-bucket or high pressure water blast means and reseal the lines by adding slipjoints or rewelding.
Efforts to solve the problem by using various solvents were essentially unsuccessful. In one case, for example, a commercial solvent was applied to a valve stem of a coke oven gas line plugged with such deposits, fifty days or longer were required before any movement of the valve stem could be noticed.
The deposits removed from the lines were in many instances relatively dry. Some areas were more dry than others in that the condensation of water and other condensables was a function of the distance from the coke plant and the ambient temperature.
It was found that certain reactions between hydrogen sulfide, hydrogen cyanide, ammonia, water and iron in the pipeline were occurring, resulting in the formation of solid deposits. These reactions also occur in the underfiring system lines feeding the coke batteries. Material removed from the underfiring gas valves was found to be very similar to the deposits found in the downstream lines. The deposits were found to contain elemental sulfur, and multivalent compounds, such as iron in different valence states.
The infrequent need to open and close valves in the underfiring lines, in all probability, permitted the buildup of deposits without being noticed. The deposition became such that it resulted in the inability to move the valve in either direction. The consequences of not being able to open or close a valve can be serious, resulting in the loss of ability to regulate or shut off gas flow.